1. Field of the Invention
The present invention relates to a sheet post-processing apparatus that is provided in an image forming apparatus for imbricately stacking sheets folded in two.
2. Description of the Related Art
Conventionally, in sheet post-processing apparatuses for stacking sheet bundles that have been saddle-stitched by a saddle stitch binding machine, many sheet post-processing apparatuses in which the sheet bundles are stacked with their folded end portions positioned downstream in a conveyance direction are discussed.
Such a sheet post-processing apparatus performs operations, as described below, to stack sheet bundles.
A discharge roller discharges a sheet bundle discharged by the discharge roller onto a conveyer belt. The conveyer belt moves the discharged sheet bundle by a predetermined amount so that a folded end portion of the discharged sheet bundle is positioned downstream in a conveyance direction of a folded end portion of a sheet bundle to be subsequently discharged. An upstream end portion (an open end portion) in the conveyance direction of the discharged sheet bundle is positioned at the bottom of a downstream end portion (a folded end portion) in the conveyance direction of the sheet bundle to be subsequently discharged.
When the sheet bundle to be subsequently discharged is discharged onto the discharged sheet bundle, the conveyer belt moves the subsequently discharged sheet by a predetermined amount downstream in preparation for discharge of a sheet bundle to be subsequently discharged. The foregoing operations are repeated so that sheet bundles discharged onto the conveyer belt are stacked with a downstream end portion in the conveyance direction of the sheet bundle overlapping with the top of an upstream end portion in the conveyance direction of the preceding sheet bundle, i.e., so-called “imbricately”, as illustrated in FIG. 17.
The sheet bundles are thus imbricately stacked for the following reason. A conventional sheet post-processing apparatus includes a stacking portion for stacking sheet bundles conveyed by a conveyer belt downstream of the conveyer belt. The reason comes from that if the length of the conveyer belt is made longer to provide a function of stacking sheet bundles for the purpose of increasing a stacking amount of the sheet bundles, the sheet post-processing apparatus grows in size.
As illustrated in FIGS. 18A and 18B, sheet bundles conveyed by a conveyer belt 750 move to a stacking portion 752 and thereby they do not receive a conveyance force from the conveyer belt 750. If the conveyer belt 750 conveys sheet bundles not imbricately, as illustrated in FIG. 18A, a folded end portion of the sheet bundle may enter an open end portion of the sheet bundle previously discharged in the stacking portion 752 to damage the open end portion, as illustrated in FIG. 18B. Even when the folded end portion of the sheet bundle does not enter the open end portion of the preceding sheet bundle, if the sheet bundle abuts on the open end of the preceding sheet bundle from a upstream side in a conveyance direction, the sheet bundle may push the preceding sheet bundle, thereby the position of the preceding sheet bundle is disarranged. However, the above-mentioned problem can be resolved by stacking the sheet bundles imbricately. Further, a stacking amount of the sheet bundles can also be increased by stacking the sheet bundles imbricately.
Such a sheet post-processing apparatus includes the conveyer belt 750 for conveying a sheet bundle discharged by a discharge roller from a downstream side in the conveyance direction, and a pressing roller 751 for pressing an upper surface of the sheet bundle on the conveyer belt 750, as illustrated in FIGS. 18A and 18B (Japanese Patent Application Laid-Open No. 09-278267).
The pressing roller 751 abuts on the conveyer belt 750 to form a nip portion, to which the sheet bundle thrusts into. The pressing roller 751 strengthens folding at a folded end of the sheet bundle while suppressing opening of an open end portion of the sheet bundle, to stack easily succeeding sheet bundles on the preceding sheet bundle, and applies a conveyance force of the conveyer belt 750 to the sheet bundles.
However, when the thickness of the sheet bundle is increased due to effects caused by, for example, the type of sheets, the grammage, or the number of sheets in the conventional sheet post-processing apparatus, it becomes difficult for the sheet bundle to thrust into the nip portion of the pressing roller 751 while a downstream end portion in the conveyance direction of the sheet bundle is overlapping with the preceding sheet bundle. It is because the sheet bundle conveyed by the conveyer belt 750 is stacked on the preceding sheet bundle and the conveyer belt 750 until it is pressed by the pressing roller 751. Therefore, the sheet bundle receives only a conveyance force generated by friction and does not receive a sufficient conveyance force until it thrusts into the nip portion. Moreover, another reason is that in the case of the sheet bundle with a large thickness, a load for the sheet bundle to thrust into the nip portion of the pressing roller 751 is increased. Therefore, if the succeeding sheet bundle cannot thrust into the nip portion only a preceding sheet bundle FT is conveyed downstream (refer to FIG. 18). After imbricate stacking is broken up, the succeeding sheet bundle, when it is conveyed downstream, may enter an open end portion of the preceding sheet bundle FT, which has passed through the conveyer belt 750, on a downstream side in the conveyance direction (refer to FIG. 18B).
In the conventional sheet post-processing apparatus, when the length of the conveyer belt 750 is made longer, and the above-mentioned stacking portion is not provided, the succeeding sheet bundle does not enter the open end portion of the preceding sheet bundle FT even if the sheet bundles are not imbricately stacked. However, when a thick sheet bundle folded in two or a thick sheet bundle even if it has no bending portion, is conveyed, the sheet bundle may be unable to thrust into the nip portion of the pressing roller 751.